Process for manufacturing an optical fiber and an optical fiber so obtained

ABSTRACT

A process for manufacturing an optical fiber includes: drawing an optical waveguide from a glass preform; applying a layer of a first coating material on the optical waveguide; curing the first coating layer material to obtain a first coating layer; applying a layer of a second coating material onto the first coating layer; applying a layer of colored coating material onto the second coating layer; curing the second coating material and the colored coating material in a single step to obtain a second coating layer superposed on the first coating layer and a colored coating layer superposed on the second coating material layer, the obtained second coating layer having an elastic modulus higher than that of the first coating layer and lower than that of the colored coating layer. An optical fiber and an apparatus for producing it are also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the field of optical fibersand of the manufacturing thereof.

2. Description of the Related Art

Optical fibers, particularly glass fibers, are manufactured startingfrom a previously made glass body, usually called “preform”, by aprocess known in the art as “drawing”. A glass preform is placed at thetop of a fiber drawing tower, where it is heated up in a furnace to atemperature sufficiently high to cause the softening of a bottom portionof the preform. The softened preform material is drawn by a tractor, toform an optical fiber glass core.

The glass core is surrounded by a layer, generally of glass, having arefractive index lower than that of the core, said layer being calledcladding. In the following, the glass core surrounded by the claddingcould be referred to as “optical waveguide”.

Over the cladding, at least one, more often two superposed UVradiation-cured coating layers are provided, which form the so-calledcoating system.

Usually, the coating system is applied onto the optical fiber coreduring the drawing process.

The coating layer which is in direct contact with the glass core iscalled “first coating” or “primary coating”; the overlaying coatinglayer, which is on the exposed surface of the coated fiber, is called“second coating” or “secondary coating”.

The coating system helps to absorb forces applied to the coated fiber,and subsequent losses associated therewith, provides protection againstmicrobending, that can lead to attenuation of the signal transmissioncapability of the coated optical glass fiber, endows the fiber with thedesired resistance to handling forces, such as those encountered whenthe coated fiber is cabled.

The first coating is usually a soft coating, having a relatively lowelastic modulus. The second coating is typically a coating having ahigher elastic modulus.

Color coding is typically used to distinguish and identify individualfibers in a complex cable.

For example, in telecommunications applications, multiple coated fiberscan be arranged into larger structures, such as ribbons and cables, tomaximize efficiency. However, after “ribboning” (i.e., arranging anumber of fibers side by side and coating them with a common coating inthe shape of ribbon) and cabling of the fiber, the individual fibershould be readily distinguishable from each other, so that they can beaccurately identified during, for example, installation and maintenance.

Although several methods can be used to color code optical fibers, colorcoding can be done advantageously with either a colored layer (typicallyas thick as about 10 microns or less), which is placed over the coatedfiber before cabling and/or “ribboning” of the same, or by applying acolored second coating onto the first coating.

The application of a colored layer can take place during the drawingprocess of the optical fiber.

Optical fiber ribbons are prepared by embedding at least two individualcolor coded fibers in a common matrix material which, like the first andsecond coatings, is also radiation-curable.

Optical fiber ribbons may comprise, e.g., 4 to 12 colored fibers. Thematrix material can encase the color coded optical glass fiber, or thematrix material can edge-bond the glass fibers together. Cure of thematrix material occurs during the “ribboning” stage, after the fibershave been color-coded by applying a colored layer.

In a ribbon design, the colored layer resides between the ribbon matrixmaterial and the fiber second coating. This means that the interfacialcharacteristics (e.g., surface energy, adhesion) of the colored layershould be carefully selected to fit with those of both the matrixmaterial and the second coating material in the ribbon structure. Inparticular, the ability of a cured matrix material to be suitablystripped off the colored layer (break-out) is an important technicalconsideration. Ribbon break-out is generally carried out by applying amechanical force, although chemical softening of the matrix with use ofsolvents is also known.

The use of a colored second coating is disclosed, for example, in U.S.Pat. No. 6,797,740.

US20040170367 relates to optical fibers including a layer of primarycoating material having a first modulus, a layer of color coatingmaterial having a second modulus, a layer of secondary material having athird modulus, and wherein the first, the second, and the third modulusvalues are different.

The layer of the primary coating material, the layer of the colorcoating material, and the layer of the secondary coating material areeach applied prior to the other layers being cured. All of the threelayers are cured together.

In an embodiment a coated optical fiber includes a layer positionedbetween the primary coating layer and the secondary coating layer. Theprimary coating layer surrounds the optical fiber (i.e. the opticalwaveguide), the colored coating layer surrounds the primary coatinglayer, and the secondary coating layer forms the outermost protectivelayer. Typically, the color coating material has a modulus between thatof the primary and secondary coating materials.

In an alternative embodiment, the color coating layer surrounds theoptical fiber and the primary coating layer is between the color coatinglayer and the outermost secondary coating layer. In this instance, themodulus of the color coating material is preferably less than or equalto the modulus of the primary coating layer. In an alternativeembodiment, the primary coating layer is adjacent the optical fiber, thesecondary coating layer is adjacent the primary coating layer, and thecolor coating layer is the outermost layer. The document provides noindication about the modulus values of the layers in this instance.

US20040179799 provides an optical fiber cable that includes a corecomprising one or more optical fibers surrounded by a coating system(therein referred to as “protective sheath”), which has aradially-varying elastic modulus, and a method for making the same. Theprotective sheath includes first and second coating layer portions basedon the same coating material. A modifier is added to the coatingmaterial of the first coating layer portion. Likewise, a modifier isadded to the coating material of the second coating layer portion. Theaddition of a modifier to the first coating layer portion and theaddition of a modifier to the second coating layer portion cause thecoating system to have an elastic modulus that varies in a radialdirection along radii extending outwardly from a center of the core ofthe cable. Different types of modifiers that can be used for thispurpose include, but are not limited to, fillers, e.g. nanoclays;cross-linking agents, e.g. acrylates; polymerization chain transferagents; photoinitiators, e.g. alpha-hydroxy ketones. The coating isprovided with different modulus by adding different amounts of modifiersor different modifiers to the coating material.

In an example, the radially-varying elastic modulus varies gradually.The radial variation could be, for example, a step-wise function suchthat the radial variation would change abruptly at some location withinthe coating.

The optical fiber may comprise an additional coating that surrounds theouter portion of the coating (three layers). The optical fiber mayfurther comprise a color layer, such as ink, for example, whichsurrounds coating layer.

SUMMARY OF THE INVENTION

The Applicant has observed that it is desirable to have an optical fiberwith the following characteristics:

-   -   coating layers with elastic modulus values radially increasing        departing from the optical fiber longitudinal axis, so as to        improve characteristics of microbending resistance;    -   a colored coating layer, for readily allowing the fiber        identification;    -   good interfacial characteristics, especially adhesion, of the        colored coating material with the second coating material, so as        to avoid detachment between colored coating layer and second        coating layer during fiber manipulation and/or under thermal or        chemical critic conditions or in connection with the stripping        of layers superposed thereto (e.g. buffer layer, ribbon matrix        layer), because the two layers constitute a substantially        monolithic entity.

The Applicant has observed that the manufacturing of a colored opticalfiber with coating layers with elastic modulus values radiallyincreasing departing from the optical fiber longitudinal axis can becumbersome.

In particular, in connection with the above discussed US 20040170367,the Applicant has observed that the disclosed process provides for thecontemporaneous curing of coating layers applied to the fiber. Such aprocess is wholly wet-on-wet, as will be explained hereinbelow, andmakes it difficult to control the diameters of the layers, especiallywhen the layers are three.

The Applicant has observed that the curing radiation provided to a layerof coating material could have an effect on the material of theunderlying layer so as to increase the curing degree thereof.

The Applicant has perceived that the colorant contained in a coloredcoating material could help to filter the curing radiation reaching anunderlying, substantially uncured coating layer. The radiation shieldingeffect of the colored coating layer could be exploited to modulate theelastic modulus by means of different curing degrees. This isparticularly advantageous when the material of the colored, upper layeris the same as the material of the second, underlying layer, differingin the presence of a colorant only.

In one aspect the present invention relates to a process formanufacturing an optical fiber, said process comprising the steps of:

-   -   drawing an optical waveguide from a glass preform;    -   applying a layer of a first coating material onto the optical        waveguide;    -   curing the first coating layer material to obtain a first        coating layer;    -   applying a layer of a second coating material onto the first        coating layer;    -   applying a layer of colored coating material onto the layer of        second coating material;    -   curing the second coating material and the colored coating        material in a single step, to obtain a second coating layer        superposed on the first coating layer and a colored coating        layer superposed on the second coating layer, the obtained        second coating layer having an elastic modulus higher than that        of the first coating layer and lower than that of the colored        coating layer.

For the purpose of the present description and of the claims thatfollow, except where otherwise indicated, all numbers expressingamounts, quantities, percentages, and so forth, are to be understood asbeing modified in all instances by the term “about”. Also, all rangesinclude any combination of the maximum and minimum points disclosed andinclude any intermediate ranges therein, which may or may not bespecifically enumerated herein.

For the purposes of the present invention, the following definitionsapply.

-   -   “Optical waveguide”: glass portion of the optical fiber        comprising a core surrounded by a cladding;    -   “Curing”: hardening of a polymer material, e.g. by cross-linking        of polymer chains, brought about by a radiation source such as        ultraviolet radiation, Electron Beam (EB) or heat.    -   “Elastic modulus” (also known as “Young's modulus”, or, in        symbols, “E”): describes tensile elasticity or the tendency of        an object to deform along an axis when opposing forces are        applied along that axis; it is defined as the ratio of tensile        stress to tensile strain (E=σ/ε).    -   “Wet-on-dry” process: multiple coating deposition wherein a        first coating material, in liquid form, is applied to the        optical waveguide; the coated optical waveguide is then passed        through a curing stage whereby the first coating material is        exposed to radiant energy to cure (and harden). A second coating        material is then applied in liquid form over the cured first        coating layer of the optical fiber. The coated optical fiber is        once again passed through a curing stage where the second        coating material is exposed to radiant energy in order to cure        (and harden).    -   “Wet-on-wet” process: multiple coating deposition wherein a        first material, in liquid form, is applied to the optical        waveguide followed by application of a second coating material,        also in liquid form, with no substantial intervening curing        stage between the application of the first coating material and        the application of the second coating material. A first coating        and a second coating result from the simultaneous curing of the        first coating material and the second coating material.

Optionally, the process of the invention comprises the step of heatingthe layer of second coating material before applying the layer ofcolored coating material. This allows achieving a better control on thefiber diameter. Said heating may be carried out by exposure to radiantenergy, for example generated by an InfraRed (IR) source or an UVsource; in the latter case, the possible curing level of the secondlayer before the application of the third colored layer should in anycase be such to ensure that the additional curing caused by theradiation filtering through the colored material during the curingthereof reaches a curing degree of the second layer of from 90% to 96%.

A degree of curing should be obtained such as to achieve an elasticmodulus of the second layer material with a value preferably equal to orlower than 1% of the elastic modulus value said material has following a100% curing. The elastic modulus values of a material after 100% curingcan be readily known by the skilled in the art, e.g. by a test or by thetechnical sheet of a marketed coating material.

The applicant observed that in a coating layer with a curing degreelower than 90% dimensional modifications, e.g. changes in the diameter,take place during the fiber life. Such a phenomenon could be due to themigration of unreacted species toward adjacent layers.

Preferably, the process of the present invention comprises curing thesecond coating layer to a degree of less than 96% during the curing ofthe colored coating material.

Preferably, the process of the present invention comprises curing thesecond coating layer to a degree higher than approximately 90% duringthe curing of the colored coating material.

Advantageously, the process of the invention comprises curing the secondcoating layer to a degree corresponding to an elastic modulus from 10%to 50% lower than the elastic modulus of the colored coating layer inthe finished optical fiber.

In another aspect, the present invention relates to an optical fibercomprising:

-   -   an optical waveguide;    -   a layer of first coating material surrounding the optical        waveguide;    -   a layer of second coating material surrounding the layer of        first coating material; and    -   a layer of colored coating material surrounding the layer of        second coating material,

wherein:

-   -   said first, second and colored coating materials have elastic        modulus values increasing departing from the optical waveguide;    -   said colored coating material has an elastic modulus of 500-1000        MPa;    -   said second coating material has an elastic modulus from 10% to        50% lower than the elastic modulus of the colored coating        material; and    -   the layer of second coating material is cured at a percentage        lower than 96%.

Preferably, the layer of second coating material is cured at apercentage equal to or higher than 90%.

Preferably, the layer of first coating material has an elastic modulusof from 1 to 2 MPa.

Preferably, said optical waveguide has a diameter of approximately 125μm.

Preferably, the layer of first coating material has a thickness of from30 to 35 μm.

Preferably, the layer of second coating material has a thickness of from20 to 35 μm.

Advantageously, the layer of the colored coating material has athickness suitable to provide mechanical resistance.

Furthermore, the colored coating material has a shielding effect whichdepends on the thickness of the layer. The thickness of the layer ofcolored coating material can be selected as a function of the colorantcontained therein. In view of the spectrum absorbance, some colorants,for example black and white, provide the colored layer with a shieldingcapacity higher than other colorant, for example yellow. For example, alayer of black-colored coating material can have a thickness smallerthan that of a layer of yellow-colored coating material while providingsubstantially the same shielding effect on the curing of the secondcoating material.

Preferably, the layer of colored coating material has a thickness offrom 10 to 15 μm.

Preferably, the second coating material is substantially the same as thecolored coating material, the latter differing in that it contains acolorant.

Advantageously, the second and the colored coating materials containsubstantially the same kind of at least one modifier, such as fillers,cross-linking agents, polymerization chain transfer agents,photoinitiators, and combinations thereof. Advantageously, said at leastone modifier is contained in the second and the colored coatingmaterials in substantially the same amount.

Using the same coating material for the second and for the coloredlayers ensures good interfacial characteristics between the two layerswith the advantages already discussed above. Using the same coatingmaterial for the second and for the colored layers eases the processingfrom an operative point of view.

Optical fibers according to the invention are advantageously used toprovide an optical fiber ribbon.

In still another aspect, the present invention relates to an apparatusfor producing an optical fiber starting from a glass preform, saidapparatus comprising:

-   -   a first applicator device for applying a layer of first coating        material onto an optical waveguide obtained from said glass        preform;    -   a first curing device for curing the layer of first coating        material;    -   a second applicator device for applying onto the layer of first        coating material cured by the curing device a layer of second        coating material and a layer of colored coating material onto        the layer of second coating material; and    -   a second curing device operable to provide an amount of radiant        energy to cure at least the layer of colored coating material.

The amount of radiant energy provided by the curing devices according tothe invention is selected in view of parameters such as drawing speed,chemical composition of the second and colored coatings, absorbance ofthe colorant in the colored coating material, thickness of the coloredcoating layer, desired degree of curing of the layer/s.

Preferably, the apparatus further comprises a heating device for heatingthe layer of second coating material before the application of the layerof colored coating material.

The heating device may comprise a third curing device.

The apparatus may comprise a first diameter measuring device before saidfirst applicator device, and/or a second diameter measuring device afterthe first applicator device and before the second applicator device,and/or a third diameter measuring device after the heating device andbefore said second curing device, and/or a fourth diameter measuringdevice after said second curing device.

The optical fiber manufactured according to the present invention is athree coating layered-optical fiber. This optical fiber exhibits goodfiber performances in terms of break-out, handling and aging in water,and shows microbending performances at least equivalent to that of theprior-art. Without being bound to a particular theory, it is believedthat said performances could be due to the fact that the layer ofcolored coating material acts as a shield against the curing radiation.Therefore, the layer of second coating material experiences lessradiation power than in the case of a two layered coating. The layer ofsecond coating material, even if formed of the same material of thecolored coating material, has a lower modulus after curing.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be bestunderstood reading the following detailed description of an embodimentof the invention, provided merely by way of non-limitative example,making reference to the annexed drawings, wherein:

FIG. 1 schematically shows an apparatus according to an embodiment ofthe present invention for manufacturing an optical fiber by a methodaccording to the present invention;

FIG. 2 shows schematically a cross section of an optical fiber, in aplane transversal to the optical fiber axis, obtained by a methodaccording to the present invention; and

FIG. 3 is a diagram showing a dependence of fiber losses (attenuation,in ordinate) on the elastic modulus (in abscissa) of the layer of secondcoating material of an optical fiber according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

Making reference to the drawings, in FIG. 1 an apparatus 100 accordingto an embodiment of the present invention is schematically depicted, forthe manufacturing of a three coating-layered optical fiber 200 by amethod according to an embodiment of the present invention.

In particular, the apparatus 100 of FIG. 1 is a vertical draw apparatus,to be mounted in an optical fiber drawing tower, suitable for awet-on-dry application of all the three layers of the three layeredoptical fiber. It is pointed out that different apparatus layouts, forexample suitable for a wet-on-wet or mixed wet-on-wet/wet-on-dryapplication of the three layers are also possible. The drawing tower andresin applicators of the wet-on-wet or wet-on-dry type are known per seand will not be described in detail.

A glass preform 105 is drawn in a furnace 110 to obtain the opticalwaveguide of the fiber 200. The fiber 200 is taken down by a tractor120. During the drawing, the diameter of the fiber 200 is measured atthe exit of the furnace 110 by a first measuring device 125. In case ofdeviation from a target diameter value (e.g., 125 μm) the firstmeasuring device 125 may send a signal to the tractor 120, which mayvary its rotational speed appropriately.

The fiber 200 is then caused to pass through a cooling tube 130, whereit is cooled down, to reach a temperature, for example, lower than 50°C., and then it is caused to pass through a first applicator device 135,having applicator dyes adapted to apply onto the glass fiber 200 a layerof a first coating material; in particular, the layer of first coatingmaterial may be applied in the form of a viscous resin.

Then, the fiber 200 coated with a layer of first coating material iscaused to pass through a first curing device 140, e.g. a UV lamp orsystem of UV lamps, which is adapted to bring about the curing of thefirst coating material; the degree of curing depends on the UV powerirradiated by the curing device 140, and/or on the fiber drawing speed.It is observed that since, in subsequent manufacturing stages (describedhereinafter) the first coating material may receive additional doses ofUV power, the curing of the first coating material by exposure to the UVpower provided by the first curing device 140 does not necessarily needto be complete.

The first curing device 140 can comprise from 1 to 4 UV lamps providingan amount of radiant energy ranging from 45 W/cm² to 150 W/cm² per lamp.

Downstream the first UV curing device 140, the diameter of the fibercoated with a layer of first coating material may then be measured by asecond measuring device 145. The measured diameter value can be used toascertain that the diameter of the optical fiber 200 is correct; in casethe measured diameter value departs too much from a target diametervalue, the optical fiber may be discarded; alternatively, the measureddiameter value can be used as a feedback to the cooling tube 130, inorder to vary the temperature to which the fiber 200 enters in the dyesof the first applicator device 135. The amount of viscous resin whichthe fiber 200 can drag, that is, the thickness of the first coatinglayer, depends on the dye geometry and on the temperature of theincoming fiber 200 with respect to that of the viscous resin. It wasobserved that measuring the diameters of the various layers of theoptical fiber 200 at the end of the manufacturing process may bedifficult, so that taking measures of the diameters of one or more ofthe layers making up the optical fiber 200 during the manufacturing maybe advantageous.

Then, the fiber 200 passes through a second applicator device 150,having applicator dyes arranged to apply onto the fiber (covered by thefirst coating layer) a layer of a second coating material; inparticular, the second coating material is applied in the form of aviscous resin.

The diameter of the fiber covered with the layers of first and secondcoating materials may then be measured by a third measuring device 160.Similarly to the second measuring device, the diameter value measured bythe third measuring device 160 can be used to ascertain that thediameter of the optical fiber 200 is correct; in case the measureddiameter value departs too much from a target diameter value, theoptical fiber may be discarded; alternatively, the measured diametervalue can be used as a feedback to the cooling tube 130, in order tovary the temperature to which the fiber enters in the dyes of the firstapplicator device 135.

Finally, the fiber 200 is caused to pass in a third applicator device165, which has applicator dyes adapted to apply onto the fiber coveredby the first and second coating materials a third layer of a coloredcoating material. The colored coating material may be in the form of aviscous resin, and includes a colorant of a type and in an amountsuitable to obtain a fiber having a desired external color.

The colored coating material may be of a same material as the secondcoating material, differing from it for the presence of the colorant;more generally, the colored coating material may be of a materialchemically compatible to the second coating material.

The colorant may be an inorganic pigment, or, more generally, a pigmentthat is chemically inert in respect to the other chemical components ofthe colored coating layer.

Then, the fiber 200 is caused to pass through a second curing device170, e.g. a UV lamp or system of lamps, which is adapted to cause thecuring of the viscous resin forming the colored coating material, thedegree of curing depending on the UV power irradiated by the secondcuring device 170, and/or on the fiber drawing speed.

The second curing device 170 can comprise from 1 to 6 UV lamps providingan amount of radiant energy ranging from 45 W/cm² to 150 W/cm² per lamp.

Due to the colorant present in the colored coating material, the coloredcoating material acts as a shield to the UV radiation. Therefore, thecontribution of the second curing device 170 to the curing of the resinforming the second coating material depends on the characteristics ofthe colored coating material, on its UV radiation shielding capability;in any case, the degree of curing of the resin forming the secondcoating material caused by the UV radiation generated by the secondcuring device 170 is reduced compared to what would be obtained shouldthe colored coating material be absent.

The diameter of the fiber covered with the three layers is then measuredby a fourth measuring device 175. Similarly to the second and thirdmeasuring devices (145, 160), the fiber diameter value measured by thefourth measuring device 175 can be used to ascertain that the diameterof the optical fiber 200 is correct; in case the measured diameter valuedeparts too much from a target diameter value, the optical fiber may bediscarded; alternatively, the measured diameter value can be used as afeedback to the cooling tube 130, in order to vary the temperature towhich the fiber enters in the dyes of the first applicator device 135.

In an embodiment of the present invention, after exiting the secondapplicator device 150, the fiber is caused to pass through a device 180,for example a third curing device, e.g. UV lamp or system of lamps,adapted to cause the viscous resin forming the second coating materialto be cured to a certain degree, depending on the UV power irradiated bythe device 180, and/or on the fiber drawing speed; in this case, thecontribution of the second UV lamp or system of lamps 170 to the curingof the resin forming the second coating material may be reduced or evennegligible. The exposure of the viscous resin forming the second coatingmaterial to UV radiation before applying the layer of colored coatingmaterial advantageously causes the heating of the second coatingmaterial, which allows to achieve a better control the final diameter ofthe optical fiber. The UV power irradiated by the device 180 may alsocause an increase of the curing level of the resin forming the firstcoating material. In some embodiments of the invention, the device 180may include, in addition or in alternative to the third curing device, aheating device like an IR source adapted to heat the second coatingmaterial.

The device 180 can comprise from 1 to 2 UV lamps providing an amount ofradiant energy ranging from 5 W/cm² to 40 W/cm² per lamp.

FIG. 2 shows schematically a cross section of the optical fiber 200manufactured following a method according to an embodiment of thepresent invention; the cross section is taken in a plane transversal tothe optical fiber axis. In the drawing, the glass optical waveguide(comprising core and cladding) of the fiber 200 is indicated with 205,the layer of first coating material is indicated with 210, the layer ofsecond coating material is indicated with 215, and the layer of coloredcoating material is indicated with 220.

The optical waveguide 205 may for example have a standard diameter ofabout 125 μm (a standard value in optical fibers for telecommunicationsapplications).

The layer 210 of first coating material has a relatively low elasticmodulus, for example of the order of 1-2 MPa in the temperature range ofuse of the optical fiber, e.g. from −30° C. to 60° C. The thickness ofthe layer 210 of first coating material may be of from 30 to 35 μm.

The layer 215 of second coating material may have an intermediateelastic modulus, higher than the elastic modulus of the layer 210 offirst coating material and lower than that of the layer 220 of coloredcoating material. Preferably, the layer 215 has a thickness of from 20to 30 μm. Preferably, the material of the layer 215 has a curing degreeof less than 96%. Advantageously, the curing degree of the material ofthe second layer 215 is higher than 90%, to avoid possible migration ofunreacted species, which may result in changes in the coating layersthickness.

The layer 220 of colored coating material may have a relatively highelastic modulus, higher than that of the layer 215 of second coatingmaterial, for example of from 500 to 1000 MPa. Preferably the thicknessof layer 220 is of about 10-15 μm. Preferably, the elastic modulus ofthe second layer 215 of second coating material is about 10-50% lowerthan that of the colored coating material. The material, e.g. the resinused for the colored coating layer may have basically the same chemicalcomposition as that of the second coating material, and containscolorant.

The external diameter of the fiber 200 may be in the range from 245 to255 μm.

The resins used for the any or each of the first, second and coloredcoating materials can be those described in U.S. Pat. No. 6,797,740. Theamount of colorant to be used may be chosen based on the thickness ofthe coating layers, as known to those skilled in the art.

The fiber produced according to the invention shows equal or evenimproved microbending resistance with respect to known fibers, thanks tothe radially increasing elastic modulus. The layer of second coatingmaterial is not the main element providing mechanical resistance of thefiber, thus it can have lower elastic modulus than the colored coatinglayer.

Moreover the colored external layer allows the fiber identification inmulti-fiber cables, without the need of a further process step ofcoloring the fiber in the cable plant.

Experimental Results

In a drawing apparatus like the apparatus 100 described in theforegoing, a glass preform having a step index profile for therefractive index has been drawn at 20 m/s. The preform has been chosento have large ratio between Mode Field Diameter (MFD) and cut-offwavelength (said ratio being hereinafter also referred to as “MAC”) inorder to emphasize the microbending sensitivity. The resulting opticalfiber turned out to have an average MFD=9.31 μm (at a wavelength of 1310nm), cabled cut-off wavelength=1138 nm and average MAC=8.18.

Two optical waveguides having a diameter of 125 μm were coated with afirst coating material (DeSolite® 6D1-78 marketed by DSM Desotech) asecond coating material (DeSolite® 3471-2-136 marketed by DSM Desotech)and a third colored material (DeSolite® 952-014, that is DeSolite®3471-2-136 plus a green colorant, marketed by DSM Desotech) saidmaterials being applied to provide the thickness values reported inTable I.

TABLE I UV power 1^(st) layer 2^(nd) layer 3^(rd) layer UV power UVpower 3^(rd) colored thickness thickness thickness Sample 1^(st) layer2^(nd) layer layer (μm) (μm) (μm) 1* 1 × 100% 1 × 100% 3 × 100% 31.522.5 10 2 1 × 100% 1 × 25% 4 × 100% 31.5 22.5 12.5

In Table I, the UV power is expressed as number of lamps×percentage ofradiation power of each lamp (100%=93 W/cm²). The first and secondcoating layers of the comparative sample 1 received an amount ofradiation such to cure the materials thereof before the application ofthe third colored layers (wet-on-dry process). The second layer ofsample 2 according to the invention received an amount of radiation ofabout 23 w/cm² such to impart a curing degree lower than 90% thusleaving the material uncured.

The microbending sensitivity of comparative Sample 1 and of Sample 2according to the invention has been tested according to IEC 62-221 IR3Ed. 1 with an expandable bobbin at room temperature. The results arereported in Table II herebelow.

TABLE II Microbending sensitivity Sample @1550 nm [dB/km/(g/mm)] 1 4.672 3.77

Sample 2 according to the invention has a significantly lowermicrobending sensitivity with respect to a standard double layered fiber(comparative sample 1). The dependence of the microbending sensitivityfrom the coating modulus (depending, in turn, from the curing degree ofthe coating material) is discussed, for instance, in F. Cocchini et al.,Journal of Lightwave Techn. 13 (1995), page 1706. The attenuationincrease of an optical fiber slightly pressed onto an external roughsurface is proportional to the ratio D/H² as shown in the followingequation:

$\begin{matrix}{{\Delta\alpha} \propto \frac{D}{H^{2}}} & {{eq}.\mspace{14mu} 1}\end{matrix}$

wherein D is the lateral rigidity of the coating system (MPa) and H isthe flexural rigidity (MPa·mm⁴). In the case of a double coating layer,the flexural rigidity H is defined as:

H=H ₀ +H ₂ =πR ₀ ⁴ E ₀+π(R ₂ ⁴ −R ₁ ²)E ₂  eq. 2

where the terms Ei are the modulus and the terms Ri are the radius ofeach component, with i=0 for the glass core, i=1 for the inner coatinglayer and i=2 for the outer coating layer, respectively.

The lateral rigidity dependence on the dual coating system can beexpressed as:

$\begin{matrix}{D_{coating} \approx {E_{1} + {E_{2}\left( \frac{R_{2} - R_{1}}{R_{2}} \right)}^{3}}} & {{eq}.\mspace{14mu} 3}\end{matrix}$

The added losses caused by the microbending do not substantially changeas a function of the modulus of the second coating layer material.

In the case of the coating system of the present invention, a third termπ(R₃ ⁴−R₂ ²)E₃ has to be added to eq. 2, because the thickness of thethird colored layer acquires relevance. As

for eq. 3 a term

${E_{3}\left( \frac{R_{3} - R_{2}}{R_{3}} \right)}^{3}$

should be added. In this hypothesis, the added losses caused by themicrobending substantially decrease as a function of the modulus of thesecond coating layer material in the region below 700 Mpa. This isreported in FIG. 3 (the following values for the various terms areassumed: 2R₀=125 μm, 2R₁=190 μm, 2R₂=245 μm, 2R₃=250 μm, E₀=72000Mpa,E₁=1 MPa, E₃=1000 MPa). The attenuation in ordinate is expressed as1/MPa·mm⁸ because, according to equation 1, it is proportional to theratio D/H², and the proportionality constant depends from the opticalcharacteristics of the waveguide and from the intensity of the lateralstress. The elastic modulus of the second coating is expressed as logMPa.

The present invention was herein disclosed making reference to someexemplary and non-limitative embodiments thereof. Those skilled in theart will readily recognize that several modifications to the describedembodiments, as well as alternative embodiments of the invention arepossible, without departing from the scope of the invention as definedin the appended claims.

1-17. (canceled)
 18. A process for manufacturing an optical fibercomprising: drawing an optical waveguide from a glass preform; applyinga layer of a first coating material onto the optical waveguide; curingthe first coating layer material to obtain a first coating layer;applying a layer of a second coating material onto the first coatinglayer; applying a layer of colored coating material onto the layer ofsecond coating material; and curing the second coating material and thecolored coating material in a single step, to obtain a second coatinglayer superposed on the first coating layer and a colored coating layersuperposed on the second coating layer, the obtained second coatinglayer having an elastic modulus higher than that of the first coatinglayer and lower than that of the colored coating layer.
 19. The processof claim 18, comprising: heating the layer of a second coating materialbefore applying the layer of colored coating material.
 20. The processof claim 19, comprising: heating said second coating material by UVradiation.
 21. An optical fiber comprising: an optical waveguide; alayer of first coating material surrounding the optical waveguide; alayer of second coating material surrounding the layer of first coatingmaterial; and a layer of colored coating material surrounding the layerof second coating material, wherein: said first, second and coloredcoating materials have progressively increasing elastic modulus valuesfrom the optical waveguide; said colored coating material has an elasticmodulus of 500-1000 MPa; said second coating material has an elasticmodulus of 10% to 50% lower than the elastic modulus of the coloredcoating material; and the layer of second coating material is cured at apercentage lower than 96%.
 22. The optical fiber of claim 21, whereinthe layer of second coating material is cured at a percentage equal toor higher than 90%.
 23. The optical fiber of claim 21, wherein the layerof first coating material has an elastic modulus of 1 to 2 MPa.
 24. Theoptical fiber of claim 21, wherein the layer of first coating materialhas a thickness of 30 to 35 μm.
 25. The optical fiber of claim 21,wherein the layer of second coating material has a thickness of 20 to 35μm.
 26. The optical fiber of claim 21, wherein the layer of coloredcoating material has a thickness of 10 to 15 μm.
 27. A optical fiberribbon comprising optical fibers according to claim
 21. 28. An apparatusfor producing an optical fiber starting from a glass preform,comprising: a first applicator device capable of applying a layer offirst coating material onto an optical waveguide obtained from saidglass preform; a first curing device capable of curing the layer offirst coating material; a second applicator device capable of applyingonto the layer of first coating material cured by the first curingdevice a layer of second coating material and a layer of colored coatingmaterial onto the layer of second coating material; and a second curingdevice, capable of providing an amount of radiant energy sufficient tocure at least the layer of colored coating material.
 29. The apparatusof claim 28, further comprising a heating device capable of heating thelayer of second coating material before the application of the layer ofcolored coating material.
 30. The apparatus of claim 29, wherein theheating device comprises a third curing device for at least partiallycuring the layer of second coating material.
 31. The apparatus of claim28, comprising a diameter measuring device before said first applicatordevice.
 32. The apparatus of claim 31, comprising a diameter measuringdevice after the first applicator device and before the secondapplicator device.
 33. The apparatus of claim 29, comprising a diametermeasuring device after the heating device and before said second curingdevice.
 34. The apparatus of claim 33, comprising a diameter measuringdevice after said second curing device.